The present invention relates to biosensors having improved sample application and measuring properties and their uses for detection, preferably, quantitative measurement, of analyte or enzyme in a liquid sample. In particular, the invention provides for a biosensor having a sample application and reaction chamber facilitating the speed and uniformity of sample application, especially small volume sample application, via capillary flow. The invention also provides for a biosensor having multiple circuits that lead to improved assay consistency and accuracy. Methods for assaying analytes or enzymes using the biosensors are further provided.
A biosensor is an analytical device that comprises at least two components: an immobilized biological component responsible for the selective recognition of the test species and a suitable transducer device responsible for relaying the biological signals for further analysis. Among others, electrochemical biosensors that employ biological recognition systems and electrochemical transudation offer a possibility of quick and real-time analysis, which is particularly suited for the rapid measurement of point-of-care industry. The evolution of these devices comes from the multi-discipline of electronics, material science, electrochemistry, biochemistry, and immunochemistry. The technology of electroanalysis is an interplay between electricity and chemistry that concerns current, potential, and charge from a chemical reaction. There are two principal types of electroanalytical measurements, potentiometric and amperometric. Potentiometric technique is a static technique with no current flow; the established potential across the ion-select membrane is measured. With different types of membrane materials, the recognition of different ions can be reached. Thus, the potentiometric probes have been widely used for directly monitoring ionic species such as calcium, potassium, and fluoride ions. In amperometric technique, an electrode potential is used to drive an electron-transfer reaction. The responsive current is measured and related to the presence and/or concentration of the target analyte. In the past, potentiometric devices have been more widely applied in clinical chemistry laboratories. But with increasing amount of research on amperometric systems in diagnostics, the balance has shifted. The amperometric biosensors make possible a practical, fast, and routine measurement of test analysts. The trend of new generations of biosensors focuses on the methodology of minimum demand of operator skills and least sample pretreatment.
Up to date, most commercially used biosensors are amperometric ones that harness redox enzymes as recognizing biocomponents and electrodes as electrochemical transducers. The mass production of inexpensive and disposable devices has been achieved recently with the help of screen-printing technology. The success in the development of these devices has led to amperometric assays for several biomolecules including glucose, cholesterol, and various drugs. This type of amperometric biosensor is typically composed of an insulating base plate, two or three electrodes, a dielectric layer, and a region for enzymatic reaction. Two-electrode biosensor consists of a working electrode, a counter electrode and a destined region where reagent for enzymatic reaction is placed. The reaction progresses when the sample liquid containing an analyte is applied onto the reaction area. Two physical effects, mesh spread and capillary action, are commonly used to guide a uniform distribution of the loaded sample on the reaction area. After the reaction is complete, the test analyte is oxidized and the electrons yielded from the reaction are trapped in a reduced co-product. A controlled-potential is then applied between the electrodes to trigger a second round of oxidoreduction. This electrical potential must be sufficient enough to drive a diffusion-limited electrooxidation at the surface of the working electrode, yet insufficient to activate irrelevant chemical reactions. After a short time of delay, the current produced by the electrochemical oxidoreduction is observed and measured and the current is correlated to the presence and/or amount of the analyte in the sample.
In the case of oxidation, oxygen is consumed in the oxidative reaction as a co-reactant and hydrogen peroxide is yielded as a co-product. Th peroxide is proportional to the concentration of analyte. Hydrogen peroxide can be detected by oxidizing it at anodic potential (e.g.,  greater than 0.6 V, Ag/AgCl) to generate an electrical signal (current). However, the potential required for oxidizing hydrogen peroxide can cause oxidation of other oxidizable chemicals such as ascorbate, bilirubin, uric acid, and the commonly used drug, e.g., acetaminophen, thus leading to an interference of electrical current to be detected. This interference can be avoided by replacing oxygen with an artificial mediator capable of transferring electrons from oxidoreductases. Several mediators have been used to enhance electron transfer between a variety of enzymes and electrodes, which include ferrocene and its derivatives, osmium complex, tetrathiofulvalene, phenazine ethosulfate, benzoquinone, and hexacyanoferrate.
In the conventional way of determining analytes in blood, a pretreatment of samples is required. A direct measurement of whole blood samples is in need of providing a simple way to save time and labor. More importantly, direct measurement of whole blood samples makes it possible for a real time monitoring for home users. For accurate measurement of a whole blood sample using an amperometric biosensor, a quick and homogenous reaction on the electrodes is essential for a successful analyte determination. The dried reagents including an oxidoreductase and a mediator have to dissolve instantly when a small volume of sample blood is applied to the biosensor. These dissolved reagents have to mix with sample blood thoroughly for the completion of the enzymatic reaction and the consistency of the subsequent electronic reaction.
The other common problems for assaying biological samples such as the whole blood are sample viscosity and the relatively large sample volume for the analysis. The whole blood sample, with its viscosity, might not be able to be distributed over sufficient reaction area. For some poorly breeding people, it might be a problem to get enough blood from a prick on fingerstick. Three types of insufficient application of blood (or other viscous samples) have been observed: first, the sample covers only the front end of the test strip; secondly, the sample covers only the right half of the strip; and thirdly, the sample covers only the left half of the strip. The insufficient or non-homogenous application of sample fluid presents a lower amount of analyte, which causes an artificial and misleading lower result.
Accordingly, there is a need in the art for biosensors and methods that provide for improved sample application and measuring properties. The present invention addresses this other related needs in the art.
In one aspect, the present invention provides for a biosensor with which the sample fluid is distributed into destined reaction area rapidly, uniformly and economically. It is another object of this invention to provide a sampling slot with least contact with reagent. Sample fluid, e.g., blood, can be loaded by a punched hole on this sampling slot and can be drawn to the reaction area quickly facilitated by an outward surface tension provided by the arcuate portion of the sampling slot and a pull-up action provided by the reaction chamber. A homogenous distribution of sample fluid can be achieved and the analyte works as a substrate to trigger the enzymatic reaction and starts the test. It is still another aspect of this invention to provide for a special design of the electrodes in a way that an elevation of electronic flow is made possible by increasing the diffusion surface between the working electrode and counter electrode. This way, the diffusion of electroactive chemicals is also uniform since both electrodes are of equal reaction areas and of the same material.
In a specific embodiment, a biosensor for electrochemical analysis of a liquid sample is provided, which biosensor comprises: a) an insulating base plate having a first end and a second end; b) an electrode system on said insulating base plate, wherein said electrode system comprises a working electrode and a counter electrode, said working and counter electrodes have conductive leads for connecting said electrodes to a readout device for electrochemical measurement on said first end of said base plate, said working electrode is engulfed by said counter electrode on all sides except the side leading to said conductive leads, and there is a gap space between said working and counter electrodes; c) a reaction area as part of said electrode system, said reaction area occupies at least a portion of said working electrode, said counter electrode and the gap space between said working electrode and said counter electrode in a direction perpendicular to said conductive leads, said reaction area is a complete cross-section of said electrode system in a direction perpendicular to said conductive leads, said reaction area is defined by covering the non-reaction-area with a layer comprising a dielectrical material, and said reaction area comprises an enzyme that catalyzes a reaction involving an analyte to be analyzed or a substrate that is involved in a reaction catalyzed by an enzyme to be analyzed; and d) a sample application and reaction chamber, wherein the bottom of said chamber is said reaction area defined in c), the top of said chamber is a cover that covers at least said reaction area, said top has an opening above said reaction area for sample application, the two side walls of said chamber in a direction perpendicular to said conductive leads are formed by said layer comprising said dielectrical material defined in c), and the two sides of said chamber in a direction parallel to said conductive leads are left open as air vents.
In another specific embodiment, a method for assaying an analyte or an enzyme in a liquid sample is provided, which method comprises: a) contacting a liquid sample containing or suspected of containing an analyte or an enzyme with the above-described biosensor in the presence of a suitable electron transfer mediator under suitable conditions whereby the analyte in the sample liquid, if there is any, is involved in a reaction catalyzed by the enzyme comprised in the reaction area of the biosensor, or the enzyme in the sample liquid, if there is any, catalyzes a reaction involving the substrate comprised in the reaction area of the biosensor, said reaction involving the analyte or substrate, in conjunction with the electron transfer mediator, leads to the generation of a current that is capable of being detected by the biosensor; and b) detecting the current generated in step a), whereby the presence or amount of the analyte or enzyme in the sample liquid is assessed.
In another aspect, the present invention provides for a biosensor by which the distribution of sample fluid can be ensured to cover all destined reaction area. The biosensor is of particular utility for use in an electrochemical sensor for measuring viscous sample fluids such as whole blood or samples containing large molecules.
In a specific embodiment, a biosensor for electrochemical analysis of a liquid sample is provided, which biosensor comprises: a) an insulating base plate having a first end and a second end; and b) an electrode system on said insulating base plate, wherein said electrode system comprises a working electrode, a counter electrode, and two reference electrodes, said working, counter and reference electrodes have conductive leads for connecting said electrodes to a readout device for electrochemical measurement on said first end of said base plate, each of said reference electrode is diagonally positioned from said working or counter electrode and there is a gap space between said working/counter, working/reference and reference/reference electrodes, said working electrode and a first reference electrode diagonally positioned from said working electrode forms a first closed circuit and said counter electrode and a second reference electrode diagonally positioned from said counter electrode forms a second closed circuit, said first and second closed circuits are connected to form a third circuit, whereby said third circuit is closed only when both said first and second circuits are closed at the same time.
In another specific embodiment, a method for assaying an analyte or an enzyme in a liquid sample is provided, which method comprises: a) providing the above-described biosensor, wherein at least a portion of the working, counter and reference electrodes and the gap space among the electrodes form a reaction area, said reaction area comprises an enzyme that catalyzes a reaction involving an analyte to be analyzed or a substrate that is involved in a reaction catalyzed by an enzyme to be analyzed; b) contacting a liquid sample containing or suspected of containing an analyte or an enzyme with the biosensor containing the enzyme or substrate in the presence of a suitable electron transfer mediator under suitable conditions whereby the analyte in the sample liquid, if there is any, is involved in a reaction catalyzed by the enzyme comprised in the reaction area of the biosensor, or the enzyme in the sample liquid, if there is any, catalyzes a reaction involving the substrate comprised in the reaction area of the biosensor, said reaction involving the analyte or substrate, in conjunction with the electron transfer mediator, leads to the generation of a current that is capable of being detected by the biosensor; and c) detecting the current generated in step a), whereby the presence or amount of the analyte or enzyme in the sample liquid is assessed.